1. Field of the Invention:
The present invention relates to the mixing of materials in the substantial absence of air. More particularly, the present invention relates to the preparation of a bone cement from a solid component and a liquid component by admixing the two components under vacuum pressure, an apparatus to effect such mixing and a kit to provide a surgeon with all the necessary materials to prepare a two-component bone cement substantially free of entrained air.
Additionally, the present invention relates to a method of preparing a bone cement from a solid component and a liquid component by admixing the two components under vacuum pressure, wherein such admixture takes place under a predetermined degree of chilling so as to control the rate of hardening of the cement, and subsequently pressurizing the admixture to inhibit entrainment of gaseous materials.
2. Description of the Prior Art:
In many orthopedic surgical procedures, it is necessary to employ a bonding material to set implants such as pins and artificial joints in bone. The cement which is employed for this purpose is generally a polymeric material which is prepared by copolymerization of its components as needed. Because of the necessity for a fairly quick setting material the cement is almost universally prepared by a surgical assistant during the course of the operation in the sterile field of the operating room. The preparation of the cement involves admixture of the cement components in a suitable reaction vessel to form a uniform polymeric reaction product. The cement is usually a (meth)acrylic material comprising a reaction product of a monomer and a polymer, e.g. methylmethacrylate monomer and polymethylmethacrylate or methylmethacrylate-styrene copolymer. In order to provide a cement which has the desired properties and which has the desired fixation of the implants, it is necessary that the compounds be uniformly and thoroughly mixed so that a homogeneous reaction product is produced. During the mixing and subsequent reaction, there are produced various vapors which may comprise a gaseous form of a volatile component or a gaseous reaction product. Because of the noxious (and toxic) nature of such vapors it is highly undesirable to be exposed to them, particularly for extended periods of time in the course of multiple preparations. Since it is necessary that the mixing be carried out for extended periods of time in order to ensure a uniform reaction product and a minimum concentration of volatile reactants, the period of exposure to harmful vapors can be substantial.
Nonetheless, despite the knowledge of the aforesaid difficulties, all known techniques for the mixing of bone cements have serious drawbacks, the most frequent being:
poor mixing, depending on the individual mixing technique; PA1 high exotherm, due to considerable amounts of monomeric component necessary to produce a workable cement mass by conventional mixing techniques; and PA1 creation of porosities, by inclusion and entrapment of air bubbles as well as by evaporation of excess monomer resulting in the degradation of the mechanical properties of the cured cement. PA1 (A) a cartridge mixer means, having an interior volume containing a first predetermined quantity of a solid bone cement component under vacuum pressure, for mixing said first predetermined quantity of a solid bone cement component with a second predetermined quantity of a liquid bone cement component in the substantial absence of air to form a third predetermined quantity of a fluid two-component bone cement; PA1 (B) container means, containing said second predetermined quantity of a liquid bone cement component, for holding a liquid bone cement component; PA1 (C) fluid transfer means, operably connectable to said cartridge mixer means and said container means, for fluidically connecting said container means and said cartridge mixer means to transfer said second predetermined quantity of a liquid bone cement component from said container means to said cartridge mixer means; PA1 (D) plug means, receivable within said container means, for automatically hermetically sealing said fluid transfer means against passage of a material therethrough upon completion of the transfer of said second predetermined quantity of a liquid bone cement component from said container means to said cartridge mixer means therethrough. PA1 cartridge means, receivable within a bone cement gun, for containing said third predetermined quantity of a fluid two-component bone cement, said cartridge means including piston means, operatively engageable by said bone cement gun and moveable within said cartridge means, for dispensing of said fluid two-component bone cement from said cartridge means; PA1 mixer means, detachably connected to said cartridge means, for agitating the contents of said cartridge means, said mixer means including inlet port means for passage of a second predetermined quantity of a liquid bone cement component into said cartridge mixer; PA1 hermetic sealing means for sealing said connected cartridge means and mixer means against the ingress of air. PA1 (A) a vacuum packed cartridge mixer, defining an interior volume, containing a first predetermined quantity of a powdery bone cement component, said cartridge mixer comprising: PA1 (B) an ampoule, defining an interior volume, containing a second predetermined quantity of a liquid bone cement component, said ampoule comprising a liquid bone cement component impermeable body member having a longitudinal axis, spout means, having a predetermined diameter, for fluidic communication with said interior volume of said ampoule and removable closure means for hermetically sealing said spout means; PA1 (C) an injector comprising PA1 (D) float means, having a specific gravity less than said liquid bone cement component, for hermetically sealingly closing said fluid passage in said resilient support means.
In order to avoid these problems, various attempts to provide vacuum mixing devices have been made.
U.S. Pat. No. 4,185,072, to Puderbaugh et al., discloses an orthopedic cement mixer which comprises a mixing vessel within an evacuable housing and mixing vanes operable from outside of the housing. The housing is provided with vacuum conduits which can be connected to a vacuum source for withdrawing vapors and/or gaseous reaction products from within the housing.
U.S. Pat. No. 4,277,184, to Solomon, discloses a disposable orthopedic implement for mixing and dispensing bone cement comprising a chamber having a barrel portion and a neck portion; a member which is reciprocable within the barrel portion of the chamber; a mixing member operatively and axially receivable within the reciprocable member; and means for rotating the mixing member within the barrel portion of the chamber.
U.S. Pat. No. 4,463,875, to Tepic, discloses an apparatus for preparing and applying a two-component cement, wherein the components are vacuum packaged in elongated flexible fluid-tight compartments and those compartments are confined in abutting relation with a seal existing around the abutting portions of the compartments. In operation, one of the compartments is gradually collapsed to force its contents to break through the abutting wall portions into the other compartment while the extension of the other compartment is controlled as it receives the contents of the one compartment. Then the other compartment is gradually collapsed to force its contents into the one compartment while controlling the extension of the one compartment. The two compartments are alternately collapsed and controlledly extended until the components therein form a homogeneous cementitious mixture. A nozzle is then attached to one of the compartments in lieu of the other compartment and the one compartment is collapsed to expel the mixture through the nozzle.
Published International patent application WO 84/03830, to Himeno, discloses an apparatus for mixing a bone cement wherein the solid component of a bone cement is sealed in a flexible bag along with a sterilizing gas. The gas is then withdrawn through a hypodermic needle to evacuate the gases within the bag. The liquid component of the bone cement is then injected into the bag, and the bag kneaded to mix the cement. After kneading the bag may be opened to remove the ready-to-use cement.
U.S. Pat. No. 4,551,135, to Gorman et al., discloses a syringe for extrusion of a semi-plastic mass. The syringe is designed so as to permit the mixing of two components of a plasticizable mixture within the syringe barrel, i.e. a liquid component may be injected into the syringe barrel containing particulate solids while venting air therefrom or a liquid component may be drawn into the syringe barrel which is only partially filled with particulate solids.
European Published patent application No. 0 178 658 (and U.S. Pat. No. 4,721,390), to Lidgren, disclose a method for producing bone cement for fixing prostheses. In order to prevent large amounts of air from being stirred into the mixture, mixing of the bone cement components occurs under vacuum. Preferably the bone cement components are mixed in a feed device from which the mixture may be pressed out, and to effect this aspect of the invention there are provided an agitator which is connectable to the feed device and a vacuum source which is also connectable to the feed device.
Published International patent application WO 86/06618, to Tepic, discloses a method of preparing a two component cement wherein the evacuated interspaces between powder component particles are flooded with liquid component, followed by mechanical homogenization of the resulting mass. The method is preferably performed in a syringe-type container whereby the liquid component in an ampoule is injected through the piston member of the syringe, and the syringe is fitted with an axially collapsible mixing element, so as to not interfere with movement of the piston for extrusion of the cement from the syringe.
U.S. Pat. No. 4,671,263, to Draenert, discloses a device for applying bone cement wherein the bone cement, prior to its application, is prepressurized at an adjustable pressure and then applied at a controllable pressure. The prepressurization suppresses bubble formation in the bone cement and the controllable pressure aids desired stratification of the bone cement around the prosthesis during application.
The brochures "MIT VAC.RTM. Vacuum Mixing System" and "ZIMMER.RTM. Vacuum Mixing System" (Copyright 1986) both disclose a bone cement mixing system providing a vacuum pump and either an evacuable mixing bowl; or an evacuable housing (with mixer), for mixing in a cartridge for a bone cement gun.
The brochure "DePuy Vacu-Mix.RTM. Orthopedic Cement Mixing System"(April 1977) discloses an open housing, for a mixing bowl, which may be connected to a vacuum pump so as to draw off vapors from above the mixing bowl.
The brochure "Stryker.RTM. Mix Evac.RTM." (copyright August 1978) discloses a disposable enclosed housing which comprises a mixing vessel within a housing, a cover for the housing and a mixing element operable through the cover. The housing may be connected to a vacuum supply to draw air through the cover and out through the housing thus preventing monomer vapors from being released to the operating room atmosphere.
The advertisement "The Simplex Enhancement Vacuum Mixer", JBJS, No. 2, 69-A, February 1987, discloses an evacuable housing, having a cover equipped with an externally operable mixing element, which will hold a bone cement gun cartridge for mixing of bone cement in the cartridge. The housing may be connected to a vacuum source to allow mixing under vacuum pressure.
The brochure "ZIMMER.RTM. Cement Centrifugation System"(Copyright 1984) discloses a system wherein after mixing of the bone cement, the bone cement is placed in a cartridge for a bone cement gun and centrifuged to help decrease cement porosity.
Numerous other devices have been conceived for the conduct of mixing operations under a controlled atmosphere or under vacuum. Illustrative of such devices are U.S. Pat. No. 2,453,914, to Hollenback, which discloses a device for mixing plaster compounds comprising a bowl, a cover for said bowl which carries an externally operated mixer for the agitation of the bowl contents and an orifice for attachment to a vacuum source to de-aerate the bowl contents.
U.S. Pat. No. 2,696,022, to Steinbock et al., discloses an investment mixer comprising a mixer bowl, a cover therefor, an agitator extending through the cover, and a fitting on the cover for connection to a vacuum source.
U.S. Pat. No. 2,973,187, to Wehmer, discloses a vacuum spatulator comprising a mixer bowl, a cover therefor, an agitator extending through the cover, and a fitting whereby the covered bowl may be placed under vacuum.
U.S. Pat. No. 3,131,912, to Steinbock, Jr., discloses an investment mixer comprising a mixing bowl, a cover for the mixing bowl, an externally operated agitator extending through the mixing bowl, and a fitting on the cover allowing evacuation of the atmosphere in the mixing bowl by connection to a vacuum source.
U.S. Pat. No. 3,343,817, to Carangelo et al., discloses an apparatus for mixing materials in the absence of air comprising a mixing bowl having an externally operated agitator in the bottom thereof and a piston receivable within the mixing bowl. The piston has an aperture therein so that, when it is lowered into place on materials to be mixed in the bowl, air caught below the piston is expelled. When the piston contacts the surface of materials to be mixed, the aperture is closed and agitation initiated.
U.S. Pat. No. 3,358,971, to Steinbock, Jr., discloses an investment mixer comprising a mixer bowl, a cover therefor, an agitator extending through said cover into said mixing bowl and an aperture for removing gases within the mixing bowl by connection to a vacuum pressure supply.
U.S. Pat. No. 3,559,961, to Bergendal, discloses an apparatus for the preparation of dental amalgams comprising a container of two halves, one of which is fitted with an outlet connectable to a vacuum pressure supply. Components to be formed into an amalgam are placed within the container, the container atmosphere is evacuated, and the evacuated container is then agitated to mix the contents thereof.
U.S. Pat. No. 3,610,586, to Price et al., discloses a dental mixing system in which prepackaged ingredients to be mixed for dental applications are stored within a container which maintains the ingredients isolated from each other prior to usage. The ingredients are intermixed by placing the container in a mixing device which rotates the base of the container while maintaining the cover stationary. A knife portion integral with the container bottom severs an isolating membrane stretched over a portion of the cover, during operation of the mixer, and permits the ingredients to mix. Through rotation of the container bottom, a homogeneous mixture is realized with a mixing paddle integral with the stationary container top or cover.
U.S. Pat. No. 3,640,510, to Lea, discloses a vacuum mixing system for dental materials comprising a closed container, an externally operated rotary stirring device therein and means for applying vacuum pressure to the contents of the closed container.
U.S. Pat. No. 4,199,866, to Drury, discloses a dental amalgamator which prevents mercury vapor, given off during amalgamation, from polluting the air in the neighborhood of the amalgamator. The amalgamator comprises a casing carrying a capsule holder and a drive means for the holder, a lid movable to a closed position in which it cooperates with the casing to form an enclosure for a capsule held by the holder, the enclosure having air inlets allowing air to flow from the atmosphere into the enclosure. A vacuum pump is arranged to communicate with the enclosure via a mercury filter medium, so that during amalgamation air is drawn into the enclosure, over the capsule, and then through the mercury filter where mercury vapors are removed.
Nonetheless, a need continues to exist for a simple two-component bone cement mixing system, which will allow mixing under vacuum but which will require little in the way of equipment. Moreover, a need also exists for a vacuum mixing system which can be readily manipulated without fear of premature loss of vacuum.